3. 4/5 - 3/8
the LCD is 40, but you can't just subtract the numerators. "fifths" and "eighths" aren't the came kind of things. In order to make them both into "fortieths" you have to make the denominators equal to 40.

The key to this kind of pronblem is to remember that multiplying by 1 does not change anything. to make 5 into 40, you need to multiply by 8. But to avoid changing the value, you multiply by 8/8 which is just 1.

4/5 * 8/8 = 32/40
3/8 * 5/5 = 15/40

Now everything is expressed in "fortieths", so you can now just subtract. 32-15 = 17, so